Communication system

ABSTRACT

An information communication system in which a switch constituting section ( 2 ) housing a data generating source is connected to a center switch constituting section ( 1 ) through a communication channel ( 3 ) using an ATM technology as if the function of an ATM switch of distributed arrangement type was realized. This information communication system flexibly handles new kinds of information provided with the progress of times, and, even when a fault occurs, properly copes with the fault.

TECHNICAL FIELD

The present invention relates to a communication system constituted by adata processing center, data generating sources and data receivingdevices and communication network linking these, in the case where thedata generating sources or data receiving devices are arranged in linearfashion.

BACKGROUND ART

The present invention relates to a communication device employed in acommunication system whereby a data processing center and datagenerating source are linked, in the case where data generating sourcesare arranged linearly; typical examples of such a communication systemin which data generating sources are arranged linearly include a roadmanagement system, railway management system, sewer management system,airfield management system, river management system, or undergroundrailway management system.

In a road management system, a management center is provided within themanagement zone and video cameras and/or vehicle sensors etc. aredisposed along the roads in the management zone, the information fromthese being collected at the management center. Also, electro-opticdisplay panels are disposed along the roads. Information concerningaccidents and/or congestion from adjacent management zones is collectedat the management center and evaluation of various types is performedusing these items of information and the information of the managementzone in question; as a result, various kinds of guidance includingcongestion information etc. are displayed on electro-optic noticeboards, thereby performing traffic management.

Taking the example of such a road management system, conventionally,information was collected by connecting an information generating sourceand the center by a one-to-one communication line, and guidance wasdisplayed on the electro-optic notice board.

Such a method suffered from the problem of lack of flexibility owing tothe following.

For example, if it was desired to add a new camera in a road managementzone, construction work had to be undertaken to lay the communicationline to the management center from the point where the addition wasmade. The same problem arose if it was desired to move a camera.

Roads constitute infrastructure that is used for a long period. New andsuperior monitoring devices make their appearance from time to time.Thus, there is the problem that construction work for layingcommunication lines arose every time a new type of device wasintroduced.

Although monitoring is performed by machines insofar as it can be,monitoring by human beings having the five senses may still benecessary. This is the case when slight changes of sound or changes inthe atmosphere etc. are to be monitored. These may necessitatepatrolling by human beings. In such cases, the monitoring staff have toperform monitoring by patrolling by carrying portable terminals. Themonitoring staff need to communicate with the center by their portableterminals from time to time. With the conventional method, preparatorywork was necessary to arrange connectors at various locations along theroad, these connectors being linked to the center by communicationlines. This was not particularly practical.

This information that it was desired to send was of various types,ranging from large amounts of information for which real-timecharacteristics were required such as video or small quantities ofinformation that did not require real time characteristics, such asmonitoring data. It is desirable for these to be integrated and sent bya number of cables if possible.

A further problem with the conventional method was that communicationbecame impossible if there was some fault in the communication device orcommunication line.

Thus, the conventional system suffered from inflexibility in regard toalterations such as addition or movement of data generating sources orinstallation of temporary data sources. It also lacked the capability ofbeing able to freely adapt to transmission data of various types such asITV image data, computer data, or audio data etc. and to send thesevarious types of data in integrated manner. Furthermore, whileself-recovery would be desirable in the event of occurrence of a fault,such a function cannot be expected with the conventional system in whichdata generating sources and the center are connected by cable.Furthermore, from the point of view of infrastructure, there are manyproblems with the conventional system in that the communication systemmust be flexibly adapted to alterations over a long period.

An object of the present invention is to realize a communication systemwherein the above problems can be solved, which has considerableflexibility, which can cope with data of various types, which can alsocope with faults, and which can flexibly cope with future improvementsin equipment.

DISCLOSURE OF THE INVENTION

The present invention is characterized by comprising: a plurality ofnode devices arranged in a distributed fashion in a plurality oflocations; one or a plurality of local communication terminals connectedto each of the node devices; communication channel means for connectingthe plurality of node devices in series; and a center device to which astarting end and a terminal end of the communication channel means areconnected; in which the center device sends data for each localcommunication terminal to the communication means and each of the nodedevices receives the data and extracts, from this data, data for a localcommunication terminal connected to the node device in question, andthereby outputs this data to the local communication terminal inquestion.

The present invention is also characterized by comprising: a pluralityof node devices arranged in a distributed fashion in a plurality oflocations; one or a plurality of local communication terminals connectedto each of the node devices; communication channel means for connectingthe plurality of node devices in series; and a center device to which astaring end and a terminal end of the communication channel means areconnected; in which the center device sends cell-formed data for eachlocal communication terminal to the communication means and each of thenode devices receives the cell-formed data, and extracts, from thereceived cell-formed data, cell-formed data for a local communicationterminal connected to the node device in question, and thereby outputsthis cell-formed data to the local communication terminal in question.

Further, each node device may comprise multiplexing transmission meanswhereby data from the local communication terminal connected to the nodedevice in question are multiplexed with data from the communicationchannel means downstream and sent to the communication channel means andthe center device may collect data from each local communicationterminal by receiving data sent to the communication channel from themultiplexing transmission means.

Also, the center device and each node device comprise a transmitter anda receiver, and perform cell transfer with the communication channelmeans through the transmitter and the receiver.

Further, alteration of logical identifier of the cells is not performedat the node devices.

Also the node devices comprise switch element means wherein cells fromthe local communication terminal and cells from the upstream centerdevice or node device are mixed without overlapping and are sent to thedownstream center device or node devices.

Cell transmission from the node devices to the communication terminal iseffected by means of a logical identifier that is permanently allocatedto the node device and the communication terminal and is effected bycopying the cells at the node device.

When a node device detects a fault by the cessation of transmission ofcells from the upstream center device or node device, it transmits afault cell bearing its own identifying information by a specifiedvirtual communication channel to the downstream center device or nodedevice.

Alternatively, a node device may constantly send with fixed timeinterval a fault monitoring cell carrying its own identifyinginformation to the downstream center device or node device and, ondetecting a fault by the cessation of transmission of the faultmonitoring cell from the upstream center device or node device, may sendfault location information to the center device.

The communication channel means may be duplicated by provision of twoindependent circuits so as to recover the communication function onoccurrence of a fault by loopback upstream and downstream of the faultlocation.

The function of an ATM switch may be constituted by the node devices,the communication channel means and the center device.

With such a configuration according to the present invention, the datagenerating sources and the center are linked by switches of distributedarrangement type, and connection between the switch-constitutingsections is achieved by the ATM technique. By this means, an informationcommunication system can be realized that has great flexibility and cancope with new information as developments take place over time. It isalso fully capable of coping with occurrence of faults.

As described above, in the present invention, a communication system isrealized by distributed arrangement of switch-constituting sections ofan ATM switch. Due to this, the following advantage are obtained.

Firstly, the switch-constituting sections may be dispersed at longdistances of several km over a wide range. Consequently, the wiring fromthe terminals to the switches can be short.

Since information exchange between switch-constituting sections iseffected by an internal format, formatting between theswitch-constituting sections is unnecessary: this simplifiesconstruction and means that less components need be used and so iseconomic.

Also, since the VPI/VCI is not replaced when the cells pass through theswitch-constituting sections, this also simplifies construction andmeans that less components need be used and so is economic.

When cells are handed over from a switch-constituting section to aterminal, this is done by copying the cells, so the cells cansubsequently flow on the transmission channel enabling multicasting andbroadcasting to be easily implemented.

When the data generating sources are arranged on a line, data collectioncan be achieved economically.

Several tens of terminals can be accommodated simply by laying one ortwo optical fibers.

A high-reliability system is achieved, wherein fault self-recovery ispossible and wherein faults can be self-detected.

Furthermore, since the ATM technique is used, compatibility with ATMexchanges is high, and this can therefore be used as the lineconcentrator system of an ATM exchange. Also, multi-media informationsuch as video, audio and data can be collected in integrated manner.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the layout of a single-layersystem according to an embodiment of the present invention;

FIG. 2 is a view showing the layout of a road monitoring systememploying the embodiment shown in FIG. 1;

FIG. 3 is a block diagram of a switch-constituting section in theembodiment shown in FIG. 1;

FIG. 4 is a block diagram of a receiving section and separating sectionof a switch-constituting section shown in FIG. 3;

FIG. 5 is a block diagram of a multiplexing section and transmittingsection of the switch-constituting section shown in FIG. 3;

FIG. 6 is a block diagram of a center switch-constituting section in theembodiment shown in FIG. 1;

FIG. 7 is a block diagram showing the layout of a double systemaccording to another embodiment of the present invention;

FIG. 8 is a diagram of loop-back on occurrence of a fault in theembodiment of FIG. 7;

FIG. 9 is a block diagram of a switch-constituting section in theembodiment shown in FIG. 7;

FIG. 10 is a block diagram of a receiving section and separating sectionof the switch-constituting section shown in FIG. 7;

FIG. 11 is a block diagram of a multiplexing section and transmittingsection of the switch-constituting section shown in FIG. 7;

FIG. 12 is a block diagram of a center switch-constituting section inthe embodiment shown in FIG. 7; and

FIG. 13 is a view showing an example of a cell identifier algorithm.

BEST MODE FOR CARRYING OUT THE INVENTION

A communication system constituted by ATM switches arranged indistributed fashion according to the present invention is described indetail below with reference to the drawings.

FIG. 1 is a block diagram showing the layout of a single-layer systemconstituting the basic system according to the present invention. Inthis Figure, 1 is a center switch-constituting section that performsoverall management of switching. 2 are switch-constituting sectionsarranged in dispersed fashion whereby signals sent from upstream flowdownstream through a transmission channel 3 and that perform exchange ofinformation with terminals through input/output ports 4. Mutualcommunication between switch-constituting sections 2 is intra-switchcommunication, so exchange of data is performed using the internalformat. Center switch-constituting section 1 corresponds to the centerdevice in the claims, switch-constituting sections 2 correspond to nodedevices in the claims, and transmission channel 3 corresponds tocommunication channel means in the claims. As will be described later,functioning as in a single ATM switch is realized by centerswitch-constituting section 1, switch-constituting sections 2 andtransmission channel 3.

Replacement of VPI/VCI is not performed in switch-constituting sections2. Since the mutual distance between switch-constituting sections 2 maybe as much as a few km, for mutual communication betweenswitch-constituting sections 2, serial transmission using an opticaltransceiver is performed.

In this way, transmission channel 3 from center switch-constitutingsection 1 is connected to the first switch-constituting section 2 andtransmission channel 3 going out from the first switch-constitutingsection 2 is connected to the next switch-constituting section 2. Inthis way, after passing successively through the switch-constitutingsections 2, transmission channel 3 finally returns to centerswitch-constituting section 1, thus constituting a loop. In addition,signal channels to terminals are provided from switch-constitutingsections 2, separately from transmission channel 3.

In order to explain the actual operation of this system, an example willbe described in which an operation of monitoring a point on a motorwayis performed by ITV and an operation of measuring wind speed by ananemometer and displaying this wind speed on an electro-optical noticeboard are performed.

FIG. 2 shows a road monitoring system in which this example is put intoeffect.

Video from ITV camera 7 is put on transmission channel 3 by aswitch-constituting section 2 and is conveyed, through centerswitch-constituting section 1, to an ITV monitor 5 connected to centerswitch-constituting section 1. When a camera direction instructionsignal and/or zoom signal etc. are input by means of an input device,not shown, attached to ITV monitor 5, these signals are conveyed to ITVcamera 7 from ITV monitor 5 by a path, not shown.

Likewise, in the case of the measurement data of an anemometer 8, thisis put on transmission channel 3 by switch-constituting section 2 and isthence conveyed through center switch-constituting section 1 to acomputer 6 connected to center switch-constituting section 1 and, afterbeing processed by computer 6, is conveyed by a route identical to thatof the instruction signal to ITV camera 7, to an electro-optic noticeboard, where the wind speed is displayed.

Thus, with such a road monitoring system, transmission of video obtainedby ITV camera 7 is frequently performed and it is anticipated that inthe future also multimedia transmission will increase even more; thesignificance of introducing an ATM switch function according to thepresent invention into a road monitoring system is therefore great.

FIG. 3 shows an example of the detailed layout of a switch-constitutingsection 2 of the basic system shown in FIG. 1.

In receiving section 21, multiplexing modules such as OC-3 or STM-1 areterminated and cells conveyed by the payload of the modules areextracted. Separating section 22 hands over cells addressed to thisswitch-constituting section 2 to a terminal, not shown.

In contrast, cells sent to multiplexing section 23 are mixed withoutoverlapping cells output from the terminal and are then conveyed by thepayload of a multiplexing module such as OC-3 or ST-1 via transmissionsection 24 and are then sent to the next-stage switch-constitutingsection 2.

As the constituent elements of switch-constituting section 2, apart fromreceiving section 21, separating section 22, multiplexing section 23,and transmission section 24 described above, there are a CPU unit 26that performs the processing at various locations of switch-constitutingpart 2, clock section 27, power source section 28 and fault alarmnotification section 25.

When CPU section 26 detects that a fault has occurred so that the signalfrom upstream has been interrupted, by free running of clock section 27,it puts the fault information from fault alarm notification section 25into the form of cells which are then transmitted downstream throughmultiplexing section 23. In this case, center switch-constitutingsection 1 that has received this can infer the location of the fault byeither putting information specific to this switch-constituting sectionin the cells or by employing a code specific to this switch-constitutingsection in the VPI/VCI that is used.

FIG. 4 shows the layout of receiving section 21 and separating section22 of the basic system. An optical signal on an optical fibre is inputto opto-electric conversion section (O/E) 211 of receiving section 21,where it is converted to an electrical signal, which is then convertedto a parallel signal by serial/parallel conversion section (S/P) 212.The OC-3 or STM-1 signal is then terminated at RX section 213 andincoming cells nsmitted by the payload are extracted .

The cells extracted by RX section 213 are sent to separating section 22of the basic system. The cells first of all enter cell filtering section221 where cells destined for the switch-constituting section 2 inquestion are extracted and sen t to the terminal. An example algorithmused for this purpose is shown in FIG. 13.

In FIG. 13, cells wherein VPI and VCI are both 0 and cells wherein VPIis 1 or more while VCI has an identifier of 0˜31 are employed forspecial purposes, or, because the identifier has already been reserved,identifiers which may be employed for user data transmission in respectof the ATM terminal may have VCI of 32 or more, VPI 1 or more, usually1˜255. Of these, the VPIs are allocated to respectiveswitch-constituting sections 2 and VCIs of 32 or more are employed forthe identification of terminals associated with respectiveswitch-constituting sections 2. Where there are two terminals associatedwith a switch-constituting section 2, these may be distinguished bywhether the VCI is odd or even. Also, specified bits of VCI or VPI maybe employed as bits for multicasting.

On the other hand, cells extracted in RX section 213 of receivingsection 21 are also sent to multiplexing section 23 in parallel withseparating section 22.

FIG. 5 shows the layout of multiplexing section 23 and transmissionsection 24 of the basic system. Cells from separating section 22, theterminal and fault alarm notification section 25 are input to switchelement section 231. Various types of construction could be adopted forswitch element section 231 such as a common buffer type or cell routingtype. Here, however, the explanation will be given in terms of abus-type switch, as illustrated. A mediation circuit 2311 at the busentrance performs cell traffic control and designates from which inputscells may be output on to the bus, thereby preventing collisions ofcells on the bus.

Fault alarm notification section 25 detects abnormality when the signalfrom upstream is interrupted and notifies center switch-constitutingsection 1 of the fact that an abnormality has occurred and the locationof the abnormality by free running of the clock and output of a faultalarm signal.

The output of switch-constituting section 231 is sent to transmittingsection 24 of the basic system. Transmission section 24 first of allputs the cells into the payload of an OC-3 or STM-1 frame in TX section241; parallel/serial conversion section (P/S) 242 then effectsconversion to a serial signal, which is then converted byelectro-optical conversion section (E/O) 243 into an optical signal,which is sent to an optical cable.

FIG. 6 shows the layout of a center switch-constituting section 1 of thebasic system.

Center switch-constituting section 1 is constituted in this example byan ATM switch 11 and network management device 12. In FIG. 6, ITVmonitor 5 and computer 6 are connected to ATM switch 11 but these arenot essential. Cells sent from a terminal through a switch-constitutingsection 2 are sent to ITV monitor 5 and/or computer 6 via ATM switch 11.When a fault alarm signal from fault alarm notification section 25 issent to network management device 12, processing for, for example,detection of the location of the fault is performed.

A description of the details of the fault location search operation ofthe basic system will now be given, including what has already beendescribed. When fault alarm notification section 25 detects that a faulthas occurred in a switch-constituting section 2, causing the signal fromupstream to be interrupted, it sends a fault alarm signal downstream inthe form of cells by allowing free running of clock section 27, fromfault alarm notification section 25 through multiplexing section 23.These cells either carry information specific to thisswitch-constituting section in their payload or a code specific to thisswitch-constituting section is employed in the VPI/VCI that is used. Afault alarm signal from fault alarm notification section 25 is sent tonetwork management device 12 where processing such as detection of thelocation of the fault is performed.

When a fault occurs, all the signals of switch-constituting sections 2downstream of that location are interrupted, so they will commence theoperation described above practically simultaneously and confusion willbe generated. Consequently, transitionally, various fault alarm signalsarrive at network management device 12. However, after a fixed time haslapsed, subsequently, a stable fault alarm signal will be received. Thereason for this is that, taking for example a switch-constitutingsection 2 at some intermediate point, although it will at first commencesending a fault alarm signal by free running of its clock oninterruption of the signal from upstream, after a certain time has elapsed, since the switch-constituting section 2 upstream of itself is ofcourse performing the same operation, there will be an incoming faultalarm signal from this. Consequently, the switch-constituting section 2in question will discontinue the sending of a fault alarm signal fromitself by stopping free running of its clock and will merely performrelaying of the fault alarm signal from upstream. When in this way thesystem has settled down to a steady condition, only theswitch-constituting section 2 adjacent the location of fault will beperforming free running of its clock and transmission of the fault alarmsignal.

Next, a double system, which is an expansion system of the basic systemwill be described. FIG. 7 shows an example of the construction of such adouble system.

The difference from the layout diagram of the basic system of FIG. 1lies in that the transmission path is duplicated. Thanks to thisduplication, if a fault occurs, as shown in FIG. 8, loop-back of thesignal is performed upstream and downstream of the location ofoccurrence of the fault, so that function recovery can be performedautomatically.

FIG. 9 shows the layout of a switch-constituting section 2 in thissystem. Since its layout is similar to that of the basic system, onlysections where there is a difference will be described.

Transmission path 3 is duplicated to provide two systems, namely, acommunication system and a loopback system. As shown in FIG. 9, thecommunication system and loopback system signal are distinguished on theinput side of separating section 22 and the output side of multiplexingsection 23, and changeover switches 210, 240 for bypassing the loopbacksystem signal are respectively provided in receiving section 21 andtransmission section 24. The loopback system signal is thereby directlyconnected to changeover switch 24 without going through separatingsection 22 and multiplexing section 23. Cells from centerswitch-constituting section 1 destined for the terminal are sent throughthe communication system and dropped at the terminal. Likewise controlis effected such that cells sent to center switch-constituting section 1from a terminal are added to the communication system in multiplexingsection 23 also, but are not added to the loopback system. In this way,under normal conditions, only the communication system is employed forcommunication; the loopback system operates, but does not performtransfer of outgoing or incoming data with a terminal; only test cellsflow through it to confirm that it is functioning properly.

If in this condition abnormality is detected, this is communicated tothe user and on-line repair is effected without adverse effect to theuser.

If a fault occurs, loopback, as will be described, is performed so thatfunctioning is recovered; however, even under these circumstances,exchange of data with the terminal is still performed by thecommunication system and, as shown in FIG. 8, the loopback system merelyrelays the signal from the fault location to the opposite side.

In the case of a switch-constituting section 2 in which loopback is notperformed, cells received from the communication system are sent to thecommunication system and cells received from the loopback system may besent to the loopback system.

Also, in the case of a switch-constituting section 2 in which loopbackis performed, cells received from the communication system may be sentto the loopback system and the cells received from the loopback systemmay be sent to the communication system. Also, fault alarm notificationsection 25 is unnecessary.

FIG. 10 shows the layout of receiving section 21 and separating section22 of the expanded system. These sections are similar in construction toreceiving section 21 and separating section 22 of the basic system ofFIG. 4, so only points of difference will be described.

Opto-electric conversion section 211 and serial/parallel conversionsection 212 of receiving section 21 are duplicated by respectiveprovision in the communication system and loopback system. A changeoverswitch 210 is provided that bypasses the loopback system signal, on theinput side of separating section 22. The signal of the loopback systemis not input to cell filtering section 221 but is directly sent tochangeover switch 240 (see FIG. 11) of transmission section 24 (see FIG.11). Parts other than these are the same in construction as receivingsection 21 and separating section 22 of the basic system shown in FIG.4.

FIG. 11 shows the layout of multiplexing section 23 and transmissionsection 24 of the expanded system. This section is similar inconstruction to the multiplexing section 23 and transmission section 24of the basic system, so only points of difference will be described.

Parallel/serial conversion section 242 and electro-optic conversionsection 243 of transmission section 24 are duplicated by respectiveprovision in the communication system and loopback system. A changeoverswitch 240 is provided that bypasses the loopback system signal, on theoutput side of TX section 241 of transmission section 24.

FIG. 12 shows the layout of center switch-constituting section 1 of theexpanded system. Only parts which are different from centerswitch-constituting section 1 of the basic system shown in FIG. 6 willbe particularly described.

The number of ports of ATM switch 11 has to be double that of the basicsystem. In addition, in loopback operation, a function is necessary totransfer cells of the loopback system that have been returned to ATMswitch 11 to the communication system on the other side.

Next, fault recovery operation of the expanded system will be describedfollowing FIG. 8.

As described above, the expanded system has a communication system and aloopback system. Cells for fault detection purposes are periodicallyallowed to flow in these two systems in order to ensure a virtualcommunication channel for fault detection purposes. If it becomesimpossible to receive these cells, this is evaluated as a fault, andoperation to shift to loopback operation for recovery of function iscommenced.

When a fault is detected, test cells are sent from centerswitch-constituting section 1 to the right-going transmission channel ofthe communication system; these are then looped back at changeoverswitch 240 (see FIG. 9) of switch-constituting section 2 adjacent centerswitch-constituting section 1, so that the test cells return to centerswitch-constituting section 1 by the left-going transmission channel ofthe loopback system. When the test cells return, thisswitch-constituting section 2 is evaluated to be normal. Test cells arethen once more sent from center switch-constituting section 1 to theright-going transmission channel of-the communication system. These passthrough switch-constituting section 2 that was previously evaluated tobe normal and are looped back at changeover switch 240 (see FIG. 9) ofthe switch-constituting section 2 which is one further on i.e. which isadjacent the switch-constituting section 2 which has now been evaluatedto be normal. If these test cells return, the switch-constitutingsection 2 is evaluated to be normal. This operation is repeated so longas the adjacent switch-constituting sections 2 are normal, until a pointof abnormality is reached; when the point of abnormality is reached, thelooping back of the last normal section is maintained and the operationis terminated.

Next, an identical operation is performed in respect of the left-goingtransmission channel and this operation is stopped, holding the loopingback of the last normal section; thus after the cells in the loopbacksystem have returned to ATM switch 11 of center switch-constitutingsection 1 they are transferred to the communication system on the otherside; thus function recovery isolating the faulty section is achieved.

As described above, the transmission channel 3 from centerswitch-constituting section 1 is connected to the firstswitch-constituting section 2 and the transmission channel 3 from thefirst switch-constituting section 2 is connected to the nextswitch-constituting section 2. In this way the transmission channel 3passes successively through switch-constituting sections 2 until itfinally returns to center switch-constituting section 1 thusconstituting a loop. In addition signal channels to the terminals,separate from transmission channel 3, are provided fromswitch-constituting sections 2. Transmission channel 3 may be duplicatedas right-going and left-going.

Cells from the terminal are mixed without overlapping with the cellsfrom transmission channel 3 in switch element section 231 withinswitch-constituting section 2 and are output onto the right-goingtransmission channel or left-going transmission channel.

Mutual transmission between switch-constituting sections 2 is effectedby serial transmission using transceivers but there is no need forconnection between the terminals etc., being internal to thedistributed-arrangement type switch, so the data format can be aninternal format which need not conform to any standard, so circuitrywhich would be needed for conversion can be dispensed with, enablingcosts to be lowered.

Furthermore, since ATM technology is employed, fine virtualcommunication channels for fault management can be set up. Specifically,communication for periodic monitoring is performed by setting up finevirtual communication channels for fault management from centerswitch-constituting section 1 on the right-going and left-goingtransmission channel 3. If a fault occurs, the occurrence of thedrop-out fault can be detected with communication for monitoringpurposes performed on either or both of the right-going and left-goingchannel. Loopback communication with center switch-constituting section1 expands outwards on both the right-going and left-going channels, sothe location of fault occurrence can be inferred as being outside themaximum expansion of the loopback communication. It is thereforeunnecessary to made an inspection for fault detection purposes over awide range.

Also, in the case where only a single transmission channel 3 isemployed, it is arranged for switch-constituting sections downstream ofa fault location to detect a fault by the non-arrival of signals, and,in response to this, to generate free running of their own clocks and tocommunicate with downstream by a fault signal carrying the number oftheir own individual switch-constituting section 2 on the fault path,discontinuing their own fault signal if a fault signal arrives fromupstream. By this means, the location of the fault can be specified inthat only the fault signal of the switch-constituting section 2 adjacentthe fault location is left.

INDUSTRIAL APPLICABILITY

The present invention is useful in communication systems constituted bya data processing center, data generating sources and data receivingdevices and a communication network linking these when the datagenerating sources and data receiving devices are arranged linearly.

What is claimed is:
 1. A communication system comprising: a plurality ofnode devices arranged in remote-distributed fashion, each node devicecomprising a switch-constituting section of an ATM exchange forexchanging ATM cells; at least one local communication terminalconnected to each of the node devices; communication channel means forconnecting the plurality of node devices in series; and a center deviceto which a starting end and a terminal end of the communication channelmeans are connected, the center device having an ATM switching functionand a node device management function; wherein each node devicemultiplexes data from the local communication terminal connected to saidnode device with transmission data sent from the center device to thelocal communication terminal over the communication channel means in aninternal format, wherein each node device extracts, from datatransmitted through the communication channel means in an internalformat, data for the at least one local communication terminal connectedto said node device and sends the extracted data to the at least onelocal communication terminal, and whereby the internal format permitsthe exchange of ATM cells without replacement of a virtual channel andvirtual path (VCI/VPI) of the ATM cells.
 2. The communication systemaccording to claim 1, wherein each node device comprises: receivingmeans for receiving ATM cells from a downstream side of thecommunication channel means; separation means for extracting, from theATM cells received by the receiving means, ATM cells for the localcommunication terminal connected to said node device and outputs theextracted ATM cells to the at least one local communication terminal;multiplexing means for multiplexing the ATM cells received by thereceiving means with ATM cells from the at least one local communicationterminal connected to said node device; and transmission means fortransmitting the ATM cells multiplexed by the multiplexing means to anupstream side of the communication channel means.
 3. The communicationsystem according to claim 2, wherein the separation means comprises:cell extracting means for extracting, from the ATM cells received by thereceiving means, the ATM cells for the at least one local communicationterminal connected to said node device; cell buffer means fortemporarily storing the ATM cells extracted by the cell extractingmeans; and cell transmitting means for transmitting the ATM cells storedin the cell buffer means to the local communication terminal connectedto said node device.
 4. The communication system according to claim 1,wherein each node device further comprises fault alarm notificationmeans which detects a fault by detecting that a signal from a downstreamside of the communication channel means is not received, generates faultalarm cells having an identification number of said node device andtransmits the fault alarm cells to an upstream side of the communicationchannel means to notify the center device of the fault.
 5. Thecommunication system according to claim 1, wherein each node devicefurther comprises fault alarm notification means which transmits cellshaving an identification number of said node device constantly to anupstream side of the communication channel means, detects a fault bydetecting that the cells from a downstream side of the communicationchannel means are not received, generates fault alarm cells having anidentification number of said node device and the transmits the faultalarm cells to an upstream side of the communication channel means tonotify the center device of the fault.
 6. The communication systemaccording to claim 1, wherein the channel means comprises twoindependent circuits so as to recover the communication function onoccurrence of a fault by loopback upstream and downstream of a faultlocation.
 7. A communication system comprising: a plurality of nodedevices arranged in remote-distributed fashion, each node devicecomprising a switch-constituting section of an ATM exchange forswitching ATM cells; at least one local communication terminal connectedto each of the node devices; communication channel means for connectingthe plurality of node devices in series; and a center device, comprisinga switch constituting section to which a starting end and a terminal endof the communication channel means are connected; wherein each nodedevice multiplexes data from the local communication terminal connectedto said node device with transmission data sent from the center deviceto the local communication terminal over the communication channelmeans, wherein each node device extracts, from data transmitted throughthe communication channel means, data for the at least one localcommunication terminal connected to said node device and sends theextracted data to the at least one local communication terminal, andwherein said node device serially transfers data received from adownstream side of the communication channel means using a virtualchannel and virtual path (VPI/VCI) to an upstream side of thecommunication channel means without changing the VPI/VCI.
 8. Acommunication system comprising: a plurality of node devices arranged inremote-distributed fashion, each node device comprising aswitch-constituting section of an ATM exchange for switching ATM cells;at least one local communication terminal connected to each of the nodedevices; communication channel means for connecting the plurality ofnode devices in series; a center device, comprising a switchconstituting section to which a starting end and a terminal end of thecommunication channel means are connected; and multiplexing meanscomprising: cell receiving means for receiving the ATM cells from the atleast one local communication terminal connected to said node device inquestion; and switch element means for multiplexing, withoutoverlapping, the ATM cells received by the receiving means from thedownstream side of the communication channel means and the ATM cellsreceived by the cell receiving means from the at least one localcommunication terminal connected to said node device, wherein themultiplexing means in each node device multiplexes data from the localcommunication terminal connected to said node device with transmissiondata sent from the center device to the local communication terminalover the communication channel means, wherein each node device extracts,from data transmitted through the communication channel means, data forthe at least one local communication terminal connected to said nodedevice and sends the extracted data to the at least one localcommunication terminal, and wherein said node device serially transfersdata received from a downstream side of the communication channel meansusing a virtual channel and virtual path (VPI/VCI) to an upstream sideof the communication channel means without changing the VPI/VCI.
 9. Acommunication system comprising: a plurality of node devices whichserially transfer ATM cells received from a downstream side of atransmission line connecting the node devices in series and using avirtual path indicator and virtual channel indicator (VPI/VCI), to anupstream side of the transmission line without replacing the VPI/VCI; atleast one local communication terminal connected to each of the nodedevices; and a center device, comprising a switch-constituting section,to which a starting end and a terminal end of the transmission line areconnected; wherein each node device multiplexes data from the localcommunication terminal connected to said node device with transmissiondata sent from the center device to the local communication terminalover the transmission line, and wherein each node device extracts, fromdata transmitted through the transmission line, data for the at leastone local communication terminal connected to said node device and sendsthe extracted data to the at least one local communication terminal.